Metals are coated with a variety of coatings for a number of purposes. For example, aluminum aircraft parts often receive an anodized coating for corrosion resistance and for enhanced paint adhesion. Anodize coatings are also used for decorative purposes. Further, titanium foil is frequently covered with a sol-gel coating when forming laminate structures where the sol-gel promotes adhesion of the various layers in the structure. Also, chromated conversion coatings prevent corrosion and promote paint adhesion when applied to aluminum and other metal.
Generally, a uniform coating amount or a coating amount within an acceptable range is desired. However, determining uniformity of the coating amount or quantifying the coating amount relative to a desired range may be difficult.
Current coating amount testing methods are destructive and therefore cannot be used with final production products. They are also time consuming, environmentally unfriendly, and disruptive to large scale production processes.
For example, current testing known in the art for amount of anodize coating or chromated conversion coating is performed by measuring the weight of a coated metallic test specimen. The coating is then chemically removed from the specimen. The metallic substrate is reweighed and the difference is the amount of the coating, which is normally given in milligrams per square foot (mg/ft2). Because the test method is destructive, it cannot be used on the manufactured product. In addition, the currently known testing process only generates a spatially averaged coating amount for the sample. As such, the currently known testing process does not determine coating amount variations over an area.
Current testing known in the art for sol-gel thickness is performed by sputtering away the sol-gel coating using depth profiling Auger spectroscopy or Glow Discharge Optical Emission Spectroscopy (GD-OES). These methods gradually remove the sol-gel coating and can determine where the sol-gel ends and the metal substrate begins. Sol-gel thickness is measured by calibrating the sol-gel removal rate and measuring the time required to remove the sol-gel layer. In addition, the currently known testing methods make very small measured spots for coating thickness on the sample. As such, the currently known testing processes require many time-consuming measured spots to determine coating thickness variations over an area.
Therefore, there is an unmet need in the art for a portable system for non-destructively determining an amount of a coating on a metallic substrate that can be used during on-line processing of the metallic substrate.